1. Field of the Invention
The present invention relates to pumps for liquids but in particular to peristaltic pumps where the liquid does not come into contact with any of the pump parts except the tube within which the liquid flows.
2. Description of the Prior Art
Heretofore, peristaltic-type pumps have been known for use in connection with pumping corrosive or sticky fluids, abrasive slurries, or fluids which cannot be contaminated by pump materials. The greatest advantage of peristaltic-type pumps is that only the substantially inert tube contacts the pumped fluid. The moving pump parts are not damaged by the pumped fluid, such as strong acids or bases, and contamination of the fluid by pump parts is not a problem. Heretofore known peristaltic-type pumps fall into one of two categories, namely the fixed guide or spring loaded guide types. FIG. 1 is an illustration of a typical fixed guide peristaltic pump. The stationary guide 10 has a circular interior configuration. The guide has two openings through which a loop of flexible tube 12 is passed and disposed within the guide. The tube is free to flex but is restrained from longitudinal motion. A rotatable pump impeller 14 is centrally located within the stationary guide. A pair of rollers 16 are mounted at each end of the pump impeller with the impeller-roller assembly having an outside diameter slightly less than the interior diameter of the stationary guide less about two times the typical tube wall thickness, whereby, the tube is pinched closed at the roller locations. As the pump impeller rotates, the pinched-off portion of the flexible tube moves circumferentially along the guide forcing fluid to move along the longitudinal axis of the tubing.
Another type of peristaltic-type pump heretofore known is the spring loaded guide type. This type of pump eliminates the need for precise tube and roller positioning, instead, the guide is pressed against the rollers with sufficient spring force to pinch the tube closed over a plurality of rollers numbering six or more. The pumping action is similar to that of FIG. 1 except that more rollers are used.
Such prior art peristaltic-type pumps have several severe disadvantages. One such disadvantage is that the prior art pumps are completely unsuited to meter flow by intermittent volume delivery, especially if the volume is small. Such prior art pumps are usually calibrated to control flow rates by adjusting the speed of rotation of the pump impeller. Delivery of predetermined volumes would require indexing the pump impeller in equal angular increments with appropriate time delays between movements. Incremental indexing to obtain predetermined volume dispensing, although possible, is complicated and expensive, and calibrating for the exact volume desired would be tedious with prior art pumps.
Another and major disadvantage relates to changes in pump performance after extended idle time in which the pump rollers have not been moved. FIG. 1 illustrates this problem. Since the roller pinches the tube closed with considerable force, the tube may be permanently deformed if the roller is left stationary for a long period of time. That depends on the age of the tube, the tube material, the fluid being pumped, and the deformation force. In prior art pumps it is not unusual for the critical time for substantially permanent deformation to be just a few hours. Permanent deformation of the tube is illustrated at depressions 18 of FIG. 1 whereat the tube is substantially permanently deformed by the pump impeller having previously been idle and stopped such that the rollers of the impeller were disposed at depressions 18. Once the deformation occurs, the pump will deliver a slightly smaller volume for each revolution than it did prior to such deformation. The volume reduction is more pronounced in pumps with closely spaced rollers and thereby more deformations relative to the stroked length of the tube. Furthermore, it is possible for the tube walls to stick together and create a substantially permanent blockage. In such a situation the pump will not prime itself and no flow will result. Still further, tube replacement is difficult and time consuming in prior art peristaltic pumps, thereby rendering pump maintenance costly.